This invention relates to a prosthetic intramedullary femoral prosthesis provided with a separate preformed sheath and to a preformed sheath for use with such a prosthesis.
Many kinds of prosthetic intramedullary femoral prostheses are held in position in the medullary canal of the femur, in which the prosthesis is implanted by bone cement. In the type of prosthesis in which there is no neck collar, the prosthesis stem can sink further into the cement after fitting and this can cause difficulties in reengagement of the tapering stem. There have been cases where the cement has been squeezed away totally during operative implantation such that there is direct contact between the metal stem and the bone, forming a window in the cement mantle. Such a window is highly undesirable, it can be the source of cracks progressing through the cement mantle and it can be a route by which debris of polyethylene, metal or bone cement can travel down the interface between the stem and the cement and out through the window, where it lodges in the bone and can be a source of bonelysis (bone death or loss of substance).
Various patents have addressed axial movement of the stem after implantation. For example, U.S. Pat. No. 4,888,023 relates to a stem having a distal tip for use in a non-cemented application with a polished outer surface. The tip is intended to be fixed to the distal part of the stem and allow for centralization and the axial sliding between the polished outer surface of the tip and the interior of the intramedullary canal.
U.S. Pat. No. 4,619,659 discloses a femoral component surrounded by a resin sheath. The entire sheath outside surface is solidly cemented into the femur along the full length thereof. The design allows sliding motion between the distal surface portions of the stem and the sheath. U.S. Pat. No. 4,670,015 relates to a hip implant having a distal tip fixedly engaging the distal portion of the stem as can be best seen in FIG. 6 thereof. European Patent Application Publication No. 0187903 Al relates to a femoral component having a distal sleeve threaded into the medullary canal, which sleeve has a threaded outer surface with an inner polyethylene sleeve to allow sliding engagement between the distal stem portion and the sheath.
U.S. Pat. No. 4,753,657 relates to a hip prosthesis having a distal sleeve fixedly attached to the distal stem. The sleeve has an outer surface which allows the subsidence of the stem within the medullary canal. U.K. Patent 1 409 053 relates to a femoral component having a distal centralizer fixedly attached to the distal stem of the prosthesis. The centralizer allows axial movement within the canal.
An article entitled "The Effects of the Collar on Total Hip Femoral Component Subsidence" by Leo A. Whiteside, David Amador and Kenneth Russell analyzes the effects of avoiding, in a non-cemented application, distal axial loading, i.e. allowing some axial motion, in a collarless prosthesis while having a tight enough fit to prevent the distal end of the prosthesis from moving in a plane perpendicular to the longitudinal axis of the femur.
An advertisement of Dow Corning Wright entitled "Whiteside Total Hip System" relates to an acrylic sleeve which centers a stem in the medullary canal and forms an integral bond to bone cement. Howmedica brochure entitled "Exeter Universal Hip System" relates to a centralizer which fits over the tip of the prosthesis, which centralizer is made from an acrylic material which is dimensioned to enclose the prosthesis stem and allow subsidence of the stem within the acrylic sleeve. A similar sleeve is shown in U.K. Patent GB 2104391.